Despite the fundamental role of Jahn-Teller effects of first and second order in shaping structural and electronic properties, there is hardly any observation in angle-resolved photoemission of solids. In oxide and halide perovskites, band structure replicas have occasionally been reported as fingerprints of Jahn-Teller effects, but no accompanying energy shifts or splittings that would allow a conclusion about their origin. In CsPbBr3, we uncover both key signatures: Upon cooling, orthorhombic replica bands emerge which had eluded earlier studies. This includes an extra valence band maximum at Γ which can even be distinguished at room temperature. Most importantly, band narrowing along Γ-M, a splitting at Γ, and the lifting of degeneracy between nonequivalent M points, all of several 100 meV, become apparent. Temperature-dependent x-ray diffraction, used as input for band structure calculations, links these effects directly to tilts and rotations of the PbBr6 octahedra. Our results uncover a strong electron-lattice interaction which is at the heart of so-far unresolved questions concerning polaronic transport, dynamic disorder, and exciton trapping in halide perovskites.

Halide perovskites are a promising class of materials for optoelectronic and photovoltaic applications, exhibiting high power conversion efficiency due to strong light absorption and long carrier diffusion lengths. While various aspects of their crystal and electronic structure have been studied, we identify a fundamental property previously overlooked that may significantly impact their efficiency. We demonstrate that halide perovskites realize a three-dimensional (3D) Lieb lattice, giving rise to a gapped 3D Dirac cone of spin-1 fermions. This leads to a fivefold reduction in effective mass compared to a conventional cubic structure and suppressed carrier backscattering due to Klein tunneling. Our conclusions are supported by band structure calculations and angle-resolved photoemission spectroscopy from CsPbBr_3 and CsSnBr_3. In particular, we reveal the transformation of the flat band of the Lieb lattice and the emergence of a dark corridor effect in photoemission from the Dirac cone, which increases as the band gap is decreased from CsPbBr_3 to CsSnBr_3.

Phosphorene, a 2D allotrope of phosphorus, is technologically very appealing because of its semiconducting properties and narrow bandgap. Further reduction of the phosphorene dimensionality may spawn exotic properties of its electronic structure, including lateral quantum confinement and topological edge states. It is demonstrated that dispersions measured along and perpendicular to the phosphorene chains self-assembled on Ag(111) reveal pronounced electronic confinement resulting in a 1D band, flat and dispersionless perpendicular to the chain direction in momentum space. Density functional theory calculations reproduce the 1D band for the experimentally determined structure. It is shown that phosphorene chains aligned equiprobably to three directions of the Ag(111) surface can be characterized by angle-resolved photoemission spectroscopy because the three rotational variants are separated in the angular domain. A semiconductor-to-metal phase transition is predicted upon increasing the density of the chain array, a promising approach to band structure engineering.

Tamai et al. discovered an unusual electronic state near the Fermi level at the interface of Cu(111) and a molecular layer of C60, which was initially attributed to C60-Cu interfacial hybridization [Phys. Rev. B 77, 075134 (2008)]. Later on, Yue et al. suggested that the state was due to the reshaping of a two-dimensional electron gas hosted at the Cu(111) surface, into an artificial graphene with Dirac cones by cutting out muffin-tin potentials of adsorbed fullerene molecules [Phys. Rev. B 102, 201401(R) (2020)]. In the present paper, we introduce a different explanation using angle-resolved photoemission and show that the observed conical bands in the C60/Cu(111) system are neither Dirac cones nor hybridization states. Rather, they are formed by umklapp scattering of photoelectrons emitted from bulk and surface bands of Cu(111) and diffracted on the (4 x 4) fullerene superstructure. The circular contours near the Fermi level, which resemble the low-energy part of graphene Dirac cones, are a result of the backfolding of the bulk band of copper, while the triangular silhouettes observed at higher binding energies, which mimic the threefold symmetric higher-binding-energy part of graphene Dirac cones, are due to an umklapp effect governed by highly coherent photoelectron diffraction of steeply dispersing Cu(111) sp-type surface resonances. We also used density functional theory to study the be-havior of the two-dimensional electron gas localized within the honeycomb net created by the electron-exclusive potentials of C60 and demonstrate doping above the Fermi level (p doping). The results show no presence of the electronic structure of artificial graphene in C60/Cu(111).

Two-dimensional (2D) Dirac materials are electronically and structurally very sensitive to proximity effects. We demonstrate, however, the opposite effect: that the deposition of a monolayer 2D material could exercise a substantial influence on the substrate electronic structure. Here we investigate TiC(111) and show that a graphene overlayer produces a proximity effect, changing the Fermi surface topology of the TiC from six electron pockets to one hole pocket on the depth of several atomic layers inside the substrate. In addition, the graphene electronic structure undergoes an extreme modification as well. While the Dirac cone remains gapless, it experiences an energy shift of 1.0 eV beyond what was recently achieved for the Lifshitz transition of overdoped graphene. Due to this shift, the antibonding pi* band at the (M) over bar point becomes occupied and observable by photoemission.

Lead halide perovskites are new key materials in various application areas such as high efficiency photovoltaics, lighting, and photodetectors. Doping with Mn, which is known to enhance the stability, has recently been reported to lead to ferromagnetism below 25 K in methylammonium lead iodide (MAPbI 3 ) mediated by superexchange. Two most recent reports confirm ferromagnetism up to room temperature but mediated by double exchange between Mn 2+ and Mn 3+ ions. Here we investigate a wide concentration range of MAMn x Pb 1− x I 3 and Mn-doped triple-cation thin films by soft X-ray absorption, X-ray magnetic circular dichroism, and quantum interference device magnetometry. The X-ray absorption lineshape shows clearly an almost pure Mn 2+ configuration, confirmed by a sum-rule analysis of the dichroism spectra. A remanent magnetization is not observed down to 2 K. Curie-Weiss fits to the magnetization yield negative Curie temperatures. All data show consistently that significant double exchange and ferromagnetism do not occur. Our results show that Mn is not suitable for creating ferromagnetism in lead halide perovskites.

Phosphorene, a 2D allotrope of phosphorus, is technologically very appealing because of its semiconducting properties and narrow band gap. Further reduction of the phosphorene dimensionality may spawn exotic properties of its electronic structure, including lateral quantum confinement and topological edge states. Phosphorene atomic chains self-assembled on Ag(111) have recently been characterized structurally but were found by angle-resolved photoemission (ARPES) to be electronically 2D. We show that these chains, although aligned equiprobably to three <$1 \bar{1} 0$> directions of the Ag(111) surface, can be characterized by ARPES because the three rotational variants are separated in the angular domain. The dispersion of the phosphorus band measured along and perpendicular to the chains reveals pronounced electronic confinement resulting in a 1D band, flat and dispersionless perpendicular to the chain direction in momentum space. Our density functional theory calculations reproduce the 1D band for the experimentally determined structure of P/Ag(111). We predict a semiconductor-to-metal phase transition upon increasing the density of the chain array so that a 2D structure would be metallic.

Artificial graphene based on molecular networks enables the creation of novel 2D materials with unique electronic and topological properties. Landau quantization has been demonstrated by CO molecules arranged on the two-dimensional electron gas on Cu(111) and the observation of electron quantization may succeed based on the created gauge fields. Recently, it was reported that instead of individual manipulation of CO molecules, simple deposition of nonpolar C-60 molecules on Cu(111) and Au(111) produces artificial graphene as evidenced by Dirac cones in photoemission spectroscopy. Here, we show that C-60-induced Dirac cones on Au(111) have a different origin. We argue that those are related to umklapp diffraction of surface electronic bands of Au on the molecular grid of C-60 in the final state of photoemission. We test this alternative explanation by precisely probing the dimensionality of the observed conical features in the photoemission spectra, by varying both the incident photon energy and the degree of charge doping via alkali adatoms. Using density functional theory calculations and spin-resolved photoemission we reveal the origin of the replicating Au(111) bands and resolve them as deep leaky surface resonances derived from the bulk Au sp-band residing at the boundary of its surface projection. We also discuss the manifold nature of these resonances which gives rise to an onion-like Fermi surface of Au(111).

The formation of large polarons has been proposed as reason for the high defect tolerance, low mobility, low charge carrier trapping, and low nonradiative recombination rates of lead halide perovskites. Recently, direct evidence for large-polaron formation has been reported from a 50% effective mass enhancement in angle-resolved photoemission of CsPbBr_{3} over theory for the orthorhombic structure. We present in-depth band dispersion measurements of CsPbBr_{3} and GW calculations, which lead to similar effective masses at the valence band maximum of 0.203±0.016 m_{0} in experiment and 0.226 m_{0} in orthorhombic theory. We argue that the effective mass can be explained solely on the basis of electron-electron correlation and large-polaron formation cannot be concluded from photoemission data.

The high efficiency of lead halide perovskite solar cells is revolutionizing photovoltaics and, at the same time, puzzling the scientific community. Here, the authors take up the prediction that a large Rashba-type spin-orbit effect slows down carrier recombination. From their experiments on organic perovskites, they conclude that this effect is much smaller than so far believed. For the inorganic CsPbBr${}_{3}$ perovskite, they probe the electron spin and reduce the upper limit for a Rashba effect by two orders of magnitude.